Electroplating method employing pulsating current of adjustable wave form



June 22, 1948. A. E. CHESTER ELECTROPLATING 2,443,599 METHOD EMPLOYING PULSATING CURRENT 0F ADJUSTABLE WAVE FORM liii lfl ilm! 2 Sheets-Sheet 1 Filed May 4, 1942 w' l ly "57555) 0 U V I 1 V E 2" 9: t a

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June 22,1948. A. E. CHESTER 9 ELECTROPLATING METHOD EMPLOYING PULSATING CURRENT OF ADJUSTABLE WAVE FORM Filgd May 4, 1942 2 Sheets-Sheet 2 copper and cadmium, while .at the same time ob- Patented June 22, 1948 UNITED STATES PATENT OFFICE ELECTROPI IATING METHOD EMPLOYING PULSATIN G CURRENT OF- ADJUSTABLE WAVE FORM Allan E. Chester, 'll'ighland Park, .Ill., assignor-to Poor & Company, Chicago, IlL, a-corporation of, Delaware Application .May 4, 1942, Serial No.'441.694

7 Claims.

taining very good uniform adherent deposits.

In particular, my invention relates to plating in still tanks while .at the same timeobtaining .high platinglspeeds greaterthan the speeds now obtainable using semi-mechanical plating processes which physically and yprogressivelymove the work, or obtainable using dynamic or projection plating processes which move the work 'and the bath relative to each other at'comparatively high speeds.

'It is important to obtain high speeds ofplating because the cost of plating-is to a very considerable extent a labor cost.

An object of my invention is to produce bright adherent metallic deposits of good quality at high electro-plating rates.

Another object of my invention is to employ :pulsating .direct'currcnt in 'electro-plating to in- ;crease"1the possible plating rate and-t0, have the instantaneous'value :oi-the pulsating applied voltgiven direct current voltage to produce a desired :plating'result.

Yet another-object of my invention is-to prorvidea method of. plating zinc which can efiec- .tively use zinc oxide containing impurities and known as secondary zinc, which iscommercially .zaviailable readily at a comparatively low price,

' .instead of requiring the pure zinc .anode which has heretofore been necessary.

My invention will be understood by reference -to the following specification 'and the. accomp'anying :drawings, wherein:

Rig; 1 shows a. circuit for combining an alterhating-current voltage of adjustablev magnitude .an'd a direct current plating voltage, and applyingthe combined resultant to the plating work,

with a storage battery floating across the plating generator, the source supplying the alternating current being connected in series with the plating generator.

Fig. 2 shows a circuit similar to that of Fig. 1

.for combining an alternating current component with the direct current output of a platinggenerator, .with. a condenser. connected acrossithe terminals of the. plating generator, the alternating :current being supplied fromagtransformer in whose primary circuita variable reactor, is ,connected in series.

- Fig. 3 shows anotherform of circuit for .com- Joining the Jdirec't. current ,output of a, plating '10 .generator with -an .al-ternatiugcunrent. ,vo'ltage component obtained from the. secondary of a transformer in whose primary. circuit is connected in series an adjustable reactor having in its magnetic circuit a separatev leg wound wit-h a winding supplied with. direct current ofxadjustable magnitude, for adjustingthe saturation of the magnetic circuit of the reactor and thereby adjusting the secondary voltage ofthetrans'former.

Fig. 4 shows a circuit for combining the direct current output of. a plating genenator'withjthe alternating current voltage component obtained from the secondary of a, shell type transformer having a third legwhich, may be adjustably saturated by a direct cur-rent winding.

Fig. 5 shows another circuitfor combining an alternating current component with the direct current output of a plating generator, the altern-ating current voltage being connected in parallel with the plating generator, and suitable reactors being inserted in series respectively between the generator and the work, and the alternating current source and thework.

Fig. 6 shows a, circuit arrangement wherein a plating tank is provided with a plurality of pairs of electrodes, and unv-arying direct current voltage .is applied to certain pairs of electrodes, and an alternating voltage is applied to certain other pairs of electrodes, so that currents flow in the tank having both direct and alternating current components. Y

Heretofore in still tank plating, with. conventional voltage supply, it has not by. any means been found at all possible to attain the theoretically possible platingspeed that would be indicated by the electro-chemical equivalents. Heretofore, if high current densities have been employed in the efiort to increase the plating speed, there has been burning on the deposit, producing dark color and poor appearancaof the deposit. Also at the high current densities, polarization occurs due to formation on the cathode of an envelope of hydrogen bubbles constituting an insulating layer and reducing the current. This hydrogen envelope has been formed to a serious extent despite the best thought-out composition of electrolyte to avoid such action.

Thus in zinc plating in still tanks, the highest current densities attainable to produce a satisfactory deposit have been of the order of 20 to 30 amperes per square foot.

These improvements, however, have not made it possible for the still tank to produce satisfactory deposits at the high speeds attainable with the projection method. Thus for zinc, even v with all the improvements which have been suggested, the minimum plating time for zinc in a still tank has been thirty minutes. In earlier zinc electro-plating processes, times up to three hours have been required. Up to the present, the application of zinc deposits has to a large extent been made by a hot dip process, rather than by electro-plating, and the use of electro-plating for zinc has been unusual because of the high cost involved due to slow plating rates, except on small parts, such as pole line hardware, for which hot dip does not produce satisfactory results.

The presence of considerable amounts of hydrogen on the cathode is also objectionable for other reasons. In alkaline baths the hydrogen is particularly likely to produce bad results on high current density, by producing large crystals of the plate which means a bad color. If the effort is made to plate steel at high current density, the quantities of hydrogen evolved are plated on to the work, and are occluded in the plate, and later work out, causing chipping, peeling and crackmg.

The prior art has used various devices in the effort to break up the hydrogen envelope, including the lifting of the work out of the bath, the useof stirring rods, and the use of wetting agents on the work.

Heretofore in the art, the use of rectifiers for electro-plating has been proposed primarily to avoid the high cost of plating generators, and the rectifiers and associated filters were accordingly designed, so far as possible, to duplicate the performance of the plating generator, that is, to give either a substantially unvarying direct current voltage, or at least to approach thereto, so far as that was possible. Another reason for trying to use rectifiers, notwithstanding their inherent shortcomings, has been to avoid the rotating parts and commutator troubles met with in plating generators. One practical objection to rectifiers is their prohibitively high costs for the heavy currents involved in plating.

One suggestion has been to employ full wave rectifiers, without any unvarying direct current component as by using a pair of rectifier tubes connected in push-pull, with filters, producing one-half of an alternating current cycle succeeded by the reversed succeeding one-half cycle.

As heretofore used, the purpose of adding the organic addition agents which have been applied to the bath, has been in the first place to reduce the crystal size of the plated metal and to produce a bright plate, and in the second place to increase the cathode efliciency, that is, the ratio between the actual rate of deposition and the electro-chemically theoretically possible rate of deposition, expressed in amperes per square foot.

The alkaline zinc baths employed in recent years have been found to have better throwing power than acid zinc baths, and for this reason, acid zinc baths have not been much used in recent years.

I superpose alternating current and direct current components to obtain a pulsating direct current, and make adjustment of their respective magnitudes to get optimum relative proportions of the alternating current and direct current components for the size and geography of the tank, that is, the positions and distance apart of the electrodes and the work, the kind of work, and other structural features involved.

I vary the magnitude of the alternating current component applied to the direct current voltage of a given magnitude, in such a way that the instantaneous value of the voltage is at all times positive. Thus, the current is at all times uni-directional. I find that if this condition is not maintained, and the direction of the current is allowed to reverse during parts of the cycle, slimes are formed and are plated as colloids, causing a smudgy deposit.

My method will produce improved deposits of a variety of metals from baths of numerous different compositions, but for best results I prefer to add to the bath a small quantity of a difficultly soluble organic salt as an addition agent.

While I usually find that my process gives best results if a particular organic addition agent is added, my process will yield good results without these addition agents.

The ordinary requirements of satisfactory field life for a plating of zinc or cadmium, is a deposit of 0.0003 to 0.0004 inch thickness, which will stand the standard test for salt spray hours. This of course varies, but for the example given a deposit of this thickness can be applied using my method and preferred electrolyte in about half a minute.

My process can be used in plating various different metals, in particular it has been found especially well adapted for metals that plate well from an alkaline cyanide bath, which usually has good throwing power.

However, myprocess can also be used for plating from other ordinarily used baths including an acid bath such as an acid sulphate zinc bath.

My method may be applied to existing installations including a standard plating generator by connecting a source of alternating current voltage of suitable characteristics into the direct current plating circuit.

One method of varying the magnitude of the alternating current component, is to employ two transformers with the primary of the second transformer connected to a variable tap on the secondary of the first transformer. This arrangement, however, involves excessive expense and is likely to spark badly even if pigtails are provided on the primary.

In Fig. 1 there is shown a circuit for controlling and applying a pulsating voltage of desired wave form with desired magnitude of alternating current component to the ordinary two electrodes of a conventional plating tank. The plating tank is shown at I, with the electrodes 2, 2. A standard plating generator 3 is connected through variable resistor 4 to one of the electrodes 2, and a storage battery 5 is floated across the generator. The secondary winding 6 of a transformer I is connected in series with the generator and the electrodes. The secondary winding 6 of transformer 1 usually consists of some three or four turns. The primary 8 of transformer I is supplied from supply terminals l0, ID, with alternating current-of standard commercial voltage and frequency, such as 230 volts, single phase, 60 cycle, and a, movable core variable inductor 9 is connected in series with the supply circuit. By adjusting the core of inductor 9 the magnitude of the alternating current component of the voltage applied to the plating circuit by the secondary 6 of the transformer 1 may be varied in the desired manner. In plating certain metals including zinc, I generally employ a standard graphite anode.

In Fig. 2 I have shown a circuit generally similar to that of Fig. 1, wherein, however, a condenser H is connected across the terminals of plating generator 3, instead of floating the secondary battery 5 of Fig. 1 thereacross. This arrangement is found preferable under some conditions.

In Fig. 3 I have shown an arrangement wherein the primary circuit of transformer 1 has connected in series therein an adjustable realctor I2 consisting of a shell type frame with three legs, having the windings l3 and M on the two outer legs connected in series between a supply terminal and a terminal of primary 8, and having an auxiliary saturating winding l5 on the middle leg of the shell frame, which is connected to a source It of direct current voltage of adjustable magnitude, for the purpose of controllably adjusting the saturation of the magnetic circuit of the shell frame, and thereby controllably adjusting the magnitude of the alternating current component of the voltage delivered by secondary windin 6. This adjustable reactor with variable direct current, for saturation, affords a very convenient and rugged means for adjusting the magnitude of the alternating current component in the platin circuit.

Another way of varying the magnitude of the alternating current component in the plating circuit is to employ a circuit generally similar to that of Fig. 1 with a single phase transformer, and to apply on one of the legs of the transformer frame an auxiliary direct current winding, and to adjust the magnitude of the current flowing therethrough for controllably saturating the magnetic circuit of the transformer and thereby adjusting its output voltage.

It has, however, been found preferable to use for such purpose, a shell type transformer frame having three legs. Such an arrangement is shown in Fig. 4, wherein secondary winding 19 of transformer i8 is connected in the plating circuit, and primary winding 20 is connected to supply terminals l0, (0. On the middle leg of the transformer frame there is connected an auxiliary direct current winding 2| which is supplied from a source 22 of direct current through variable resistor 23. By adjusting variable resistor 23, the magnitude of the saturating direct current passing through auxiliary winding 2| may be controlled, thus adjusting the saturation of the magnetic circuit of transformer l8 and hence varying the magnitude of the alternating voltage delivered by secondary winding l9 and applied as the alternating current component in the plating circuit. In the transformer shown in Fig. 4, it is also usually convenient to constitute secondary winding IQ of only three or four turns.

In Fig. 1-4 I have shown circuit arrangements wherein the source of alternating current has been connected in series with the source of direct current constituted by the plating generator.

In Fig. 5 there is shown a circuit arrangement alternating current component is conveniently the secondary winding of a transformer 29, having primary winding 3| which is connected in series to a source of commercial alternating current preferably through an adjustable inductor 3-2. This parallel circuit arrangement may in some cases be found desirable to avoid any possible feed-back, but such feed-back seldom presents any problem. In using the circuit of Fig. 5, it is necessary by trial to select the proper adjustment value of inductor 21. The arrangement of Fig. 5 usually involves a more expensive type of tank construction than do the arrangements of Figs. 1-4.

It is also possible to use a. separate cathode and anode for applying the unvarying direct current voltage to the bath, and to employ a separate pair of auxiliary electrodes for applying the alternating voltage of desired magnitude to the bath, suitably positioning the anode and cathode in spaced relation to the auxiliary electrodes.

Such an arrangement is shown in Fig. 6, wherein there are positioned Within plating tank 49, a plurality of pairs of direct-current electrodes, 34, 35, 35, 31, 38, 3-9, and also a plurality of pairs of alternating current electrodes 40, 4|, 42, 43. Objects to be plated'constituting the work may constitute the cathodes of the direct current electrodes. The'direct current electrodes are connected to a plating generator 44. The alternating currentelectrodes are connected to the secondary winding 41 of transformer 45, whose primary 48 is supplied from a commercial source of alternating current through a variable inductor 48. This arrangement inherently in volves more expensive wiring connections to the tank and a great deal more copper to get both the alternating current and the direct current separately to the bath, than do the arrangements of Figs. 1-4, and the arrangement of Fig. 6 is ordinarily employed only if serious difiicuities are encountered with feed-back, or if bad sparking is encountered on the commutator of the plating generator, which difficulty seldom arises.

I usually ground the anode to avoid the possibility of having a comparatively high primary voltage such as 230 volts to ground.

It is also possible to employ an alternator instead of a transformer for supplying the alternating current component, if such source is con veniently available.

The alternating current voltage may be capacitatively coupled to the plating circuit if special requirements exist, but this is usually more expensive than the method which I have described in detail for applying the alternating current to the plating circuit.

For supplying the alternating current component for my method, it would also be possible to use one or more rectifiers, as of the selenium or copper type, but for the type of load involved in electro-plating employing my method, these have been found to give unsatisfactory and poor performance including poor voltage regulation and to involve comparatively high costs. Furthermore, rectifiers do not conveniently provide for a variable adjustment of the magnitude of the alternating current component of the pulsating voltage which is necessary to obtain the full benefit of my process.

In the arrangement of Fig. 1 and other figures, the transformer reactors, 'wiring, meters, resistors, and protective devices may be conveniently mounted on a frame as a panel unit.

all of these circuits 1 provide adjusting means for varying the magnitude of the alternating current component which is combined with the direct current voltage component.

For a given value of direct current voltage from the usual plating generator, I adjust the value of the alternating current voltage com-ponent in such a manner that the lowest instantaneous value of voltage representing the algebraic sum of the constant direct current potential and the greatest negative value of the alternating. current voltage wave at its negative peak is positive but reduced to such a value that polarization at the cathode is practically nonexisting, and no hydrogen bubbles are maintained on the cathode at the instant of the low peak. This condition will generally occur for baths such as an alkaline cyanide zinc bath, to which it is usual in conventional plating to apply a direct current voltage of about three volts. between 0.5 and 1.5 volts for the valley" instantaneous value in using my method. A satisfactory lowest instantaneous voltage is in many cases about 0.9 ,volt.

Thus if a plating generator producing 6 volts direct current across its terminals on full load is connected in series with the output winding of a transformer which delivers an alternating current voltage of 3.6 volts R. M. S. at its terminals with -full load current passing therethrough, the minimum instantaneous val1ey" voltage will be 6-5.09 or 0.91 volt.

Within this indicated range, the adjustments are made to provide a pulsating voltage whose minimum instantaneous valley value has been selected as an optimum value, determined according to the shape and dimensions of the tank and of the work. The area of the work constituting the cathode is an important factor, since with the anode area being fixed, the cathode area determines the anode-cathode ratio.

The unvarying direct current component which I employ may be permitted to go as high as 24 volts, while still keeping the minimum instantaneous value of the pulsating current produced down to from 0.5 to 1.5 volts, by supplying an alternating current component of suitably large magnitude.

I usually find that it is preferable to employ a sinusoidal alternating cur-rent voltage whose peak value, considering the alternating current voltage separately, is at least one-half of the unvarying direct current voltage. In general, I find that the value of the alternating current voltage component might be a little less, when employing danglers" in barrel plating, than in other forms of plating.

Current may also be used whose alternating current component contains harmonics.

In ordinary cases, I have found it satisfactory to employ 60 cycle alternating current, to supply the alternating current component of my pulsating'direct current voltage. However, I have also employed with satisfactory results, alternating current of other commercial frequency,

such as 25 cycles. I have also found alternating current of other frequencies to be advantageous under certain conditions.

I have found that my method of plating is suitable for employment with a variety of difierent kinds ofplating baths, including:

A. Alkaline zinc solution B. Alkaline copper solution C. Alkaline cadmium solution D. Acid zinc solution E. Silver solutions F. Arsenic-antimony alloy solutions.

Grams per liter Sodium cyanide '15 Zinc oxide Sodium hydro e 15 To the above bath there are added:

of 1% by weight (i. e., 1 gram per liter) of lauric alcohol sulfonate (lauric sulfonate); 10 grams per liter of calcium gluconate.

After these ingredients have been mixed in the water to constitute the bath, they are continuously agitated with a rotating paddle for five or six hours to get complete reactions, and all of the ingredients are dissolved except the gluconate which is held in suspension.

The lauric sulfonate assists materially in retaining the glucona'te in suspension. The lauric sulfonate may be considered to be a wetting. suspending and floccul-ating agent, reducing the surface tension and producing better deposits, by reason of its being occluded in the plate at the cathode. As a practical matter it is necessary for the bath to contain some agent capable of retaining the gluconate in suspension.

In practice, the quantities of the several ingredients as mentioned for the preferred bath may be varied within tolerances of approximately plus or minus 20% without serious impairment of the results obtained. After the bath is originally prepared according to the formula above given, it may be expected that its composition will vary considerably after it has been used in active piating work for a number of weeks, varying in concentration perhaps as much as but it will continue to give satisfactory plating results. Plating with this bath is ordinarily conducted at ordinary room temperatures.

I have discovered that the functions of both calcium sulphate and the glucose can be combined by adding to the plating bath quantities of calcium gluconate, which, only slightly soluble, acts in the same fashion as calcium sulphate to control the carbonate content of the solution by the precipitation of calcium carbonate, and also acts to improve theappearance of the plate and the cathode efliciency of the electrical circuit. In other words, there are three functions of the calcium gluconate in the bath:

(1) To decrease the sodium carbonate content by the precipitation of insoluble calcium carbonate.

(2) To improve the appearance of the plate without the further addition of invert organic sugars or glucosides, which are commonly used as brightening agents.

(3) To act as a protective colloid which increases the cathode efiiciency appreciably.

The calcium gluconate acts chemically in the following manner:

In solution to some slight extent, the acid ion oxidizes at the anode, due to the presence of nascent oxygen. This generates saccharic acid, which, in turn, reacts with the excess sodium carbonate to produce the insoluble calcium carbonate, and return the acid ion to the anode for regeneration as a saccharate.

Thus the calcium gluconate acts as a control agent for controlling the action and performance of the described bath when employed with the pulsating current method which I have described.

The use of the pulsating current of the characteristics which I have described, relieves the applied electric field pressure which applies the potential to the hydrogen ions tending to keep them on the cathode, and thus breaks up the hydrogen envelope which would normally form on the cathode under heavy current density.

I find that use of my process and my electrolyte produces plating results faster in a still tank than can be obtained by employing any of the standard mechanical plating processes involving successive plating steps.

When my method and preferred bath are used with a graphite anode which is my preferred procedure, it is necessary from time to time to add zinc to the bath to restore normal zinc content.

While my pulsating current may be employed at high current densities with a conventional known formula of plating bath, it is usually found after two or three hours that the metal ion concentration has been reduced below a suitable value to produce good results.

The combination of my pulsating current method with optimum adjustment, and my preferred addition agent, work together in an unusual and unexpected manner to produce a very rapid plating of a clean adherent satisfactory deposit.

My method including my preferred pulsating voltage and my preferred electrolyte, produces extremely rapid and clean adherent deposits in a manner not heretofore attained in the art. The bath is kept free from sludge and the deposits are brlgh The apparatus is simple and rugged, and the application of the method does not involve difficult adjustments.

It will be obvious that the method and apparatus which I have described are susceptible of modifications which will be evident to those skilled in the art, and all such modifications as are included within the scope of the appended claims I consider to be comprehended within the scope of my invention.

I claim:

1. [In electro-plating of work employing an aqueous bath containing dissolved salts of the plating metal and an anode and wherein the is the cathode, the method of obtaining napid clean deposition which consists in applying to the cathode with respect to the anode a pulsating direct current voltage whose instantaneous value is always positive, said pulsating voltage having an unvarying direct current component and further having a sinusoidal alternating current component, the lowest instantaneous value of the said pulsating voltage at the negative alternating peak being between 0.5 volts and 1.5 volts, said direct current component having a voltage not substantially higher than 24 volts and said alternating current component having a frequency within the range of from about 25 to 60 cycles, inclusive.

2. In electro-plating of work employing an aqueous bath containing dissolved salts of the plating metal and an anode and wherein the Work is the cathode, the method of obtaining rapid clean deposition which consists in applying to the cathode with respect to the anode a pulsating direct current voltage whose instantaneous value is always positive, said pulsating voltage having an unvarying direct current com nent and [further having a sinusoidal alternating current component, the instantaneous peak value of the alternating component considered separately being at least half of the value of the unvarying direct current component and the lowest instantaneous value of the said pulsating voltage at the negative alternating peak being between 0.5 and 1.5 volt, said direct current component having a voltage not substantially higher than 24 volts and said alternating current component having a frequency within the range of from about 25 to 60 cycles, inclusive.

3. In electro-plating of work employing an aqueous alkaline zinc bath, and further employing an anode, and a cathode constituting the work, the method of obtaining rapid deposition which consists in applying to the cathode with respect to the anode a pulsating direct current voltage whose instantaneous value is always positive, the lowest instantaneous value of said pulsating voltage at the negative alternating peak being between 0.5 volt and 1.5 volts, said direct current component having a voltage not substantially higher than 24 volts and said alternating current component having a frequency within the range of from about 25 to 60 cycles, inclusive.

4. In the electroplating of metals employing an aqueous bath containing dissolved salts of the plating metal provided with an anode and wherein the work to be electroplated is the cathode, the method of electrodeposition which comprises electnodepositing stantaneous value of the voltage of which is always positive, said pulsating current having a direct current voltage component and an alternating current voltage component, the lowest instantaneous value of the said pulsating voltage at the negative peak being between 0.5 volt and 1.5 volts, said direct current component having a voltage not substantially higher than 24 volts, said alternating current component, when considered separately, having an instantaneous peak value of substantially at least half of the value of the direct current component, and said alternating current component having a frequency within the range of from about 25 to 60 cycles, inclusive.

5. In the electrodeposition of zinc, the step which comprises electrodepositing zinc from an aqueous alkaline cyanid-zinc plating bath with a pulsating current voltage in sine wave form, the instantaneous value of which is always positive, said pulsating voltage having a direct current component and an altemating current component, the lowest instantaneous value of the said pulsating voltage at the negative peak being between 0.5 volt and 1.5 volts, said direct current 11 component having a. voltage not substantially higher than 24 volts andsaidalternating current component having a frequency within the range of 'from about 25 to 60 cycles,inclusive.

6. In the electrodeposition of Zinc, the step which comprises electrodepositing zinc from an aqueous acidzinc plating bath with a pulsating current voltage in sine wave form, the instantaneous value of which is always positive, said pulsating voltage having a direct current component and an alternating current component, the lowest instantaneous value of the said pulsating voltage at the negative peak being between 0.5 volt and 1.5 volts, said direct current component having a voltage not substantially higher than 24 volts and-said alternatingcurrent component having a frequency within the range-of from about 25 to 60 cycleainclusive.

7. In the electrodeposition of copper, the step which comprises electrodepositing copper from. an aqueous alkaline copper plating bath witha pule sating current solution in sine wave form, the instantaneous value of which is always positive, said pulsating voltage having a direct current component and an alternating current component, the lowest instantaneous value of the said pulsating voltage at the negative peak being be- 12 tween 0.5 volt and 1.5 volts, said direct current component having a voltage. notsubstantially higher than 24 volts and said alternating current component having a frequency within the range of from about 25 to cycles, inclusive.

ALLAN E. CHESTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,388,874 Mershon Aug. 30, 1921 1,755,479 Jones Apr. 22, 1930 1,918,605 Jones July 18, 1933 2,063,760 Sohulein Dec. 8, 1936 FOREIGN PATENTS Number Country Date 414,939 Great Britain Aug. 16, 1934 164,709 Germany Aug. 23, 1928 OTHER REFERENCES Metal Industry, Apr. 19, 1929, pages 396-398.

Transactions Faraday Society, vol. 18 (1922), pages 102-111; and vol. 24 (1928), pages 348-358. 

